1. Field of the Invention
The invention relates to the field of micro-optical circuits fabricated by micromachining techniques, and in particular, to a free-space integrated optical pickup head for optical data storage.
2. Description of the Prior Art
Integrated optics has been an active research area since it was proposed first in 1969, C. S. E. Miller, "Integrated Optics: An Introduction," Bell Systems Technology Journal, Vol. 48, No. 7, pages 2059-68 (1969). Most of the research efforts in integrated optics has been on guided wave systems. See, for example, H. Nishihara et al., "Optical Integrated Circuits," McGraw Hill 1985; and T. Tamir, "Guided Wave Optoelectronics," Springer-Verlag (1990). Free-space optics offers many unique advantages that cannot be achieved in guided wave devices. For example, free-space optics allow three-dimensional optical interconnections that could significantly improve the communication bottlenecks in very large semiconductor integrated systems. Much higher spatial bandwidth can also be achieved in free-space optics as opposed to guided wave devices. It is also possible to perform sophisticated optical information processing, such as a Fourier transforms, in a free-space system using lenses.
However, most of the present free-space optical systems are comprised of bulk optical elements or multiple plane micro-lens arrays that cannot be integrated on a single chip. They are, instead, discrete microsystems. Micromachining of silicon substrate has been applied to miniature optical bench and integrated optical systems since the 1970's. See K. E. Petersen, "Silicon as a Mechanical Material," Proc. IEEE, Vol. 70, No. 5, at 420-57 (1982). Silicon V-grooves in crystalline wafers have been employed in many products for aligning optical fibers. Torsional mirrors have been used in digital micro-mirror devices for projection displays.
Micromachining allows an inexpensive and reproducible batch processing of optical components. However, to date, most of the micro-optical components and systems are designed for optical access normal to the substrate surface because the microfabricated optical elements are confined to and within the surface of the substrate. Such examples include digital micro-mirrors as described by L. J. Hornbeck, "Deformable-Mirror Spatial Light Modulators," Proc. SPIE, Vol. 1150, pp 86-102 (1990), and microfabricated optical choppers, M. T. Ching et al., "Microfabricated Optical Chopper," Proc. SPIE, Vol. 1992 at 40-46 (1993). Since external optics is needed for optical systems using components restricted within the substrate surface, monolithic integration of the complete optical system on a single chip is not possible with these technologies.
Monolithic integration of a whole optical system or a micro-optical system, drastically reduces its size, weight and cost. Furthermore, expensive packaging process of individual optical components is totally eliminated. One key component for a monolithic micro-optical system is an out of plane optical element, that is optical elements which are not confined to the surface of the substrate. In particular, vertical optical components standing perpendicularly to the surface allow multiple elements to be cascaded along the optical axis on the substrate. The LIGA process can produce tall structures with a height of several hundred micrometers and can be used to fabricate optical elements, although X-ray lithography is required to do so. However, it is very difficult to create patterns on the sidewalls of the tall structures fabricated by LIGA methodologies. These patterns will perform optical functions such as lenses or gratings.
More recently, a micro-hinged technology has been developed which allows three-dimensional structures to be assembled from thin plates on the surface of the substrate. See K. S. J. Pister et al., "Microfabricated Hinges," Sensors and Actuators A, Vol. 33, at 249-56 (1992). This technology is based on surface micromachining and is compatible with most micro-actuator fabrication processes.
Three-dimensional micro-optics fabricated by surface micromachining opens a new area for integrated optics in free-space. Using this technology, integrated micro-optical elements can be patterned by conventional photolithography and etching techniques and then made to stand perpendicular to the substrate. Thus, multiple free-space optic elements along the optical path can be made on the same substrate. The substrate serves as a micro-optical bench and lenses, mirrors and other components are prealigned by photolithography and then constructed by microfabrication. Micro-Fresnel lenses, standing perpendicular to the substrate, have been fabricated, micro-mirrors, gratings, beam splitters and lens mounts have also been demonstrated. The microfabrication technique is compatible with microactuators and the integration of a vertical mirror with a rotational stage has been demonstrated. Using surface micromachined free-space structures thus presents yet unrealized opportunities for the fabrication of optical systems.
Currently, optical data storage on compact disks, laser disks, and magento-optic disks as well as other optical medium, is emerging as the most promising technology for mass digital storage with removable storage media. However, in each of these systems, an optical pickup head is necessary to read or write onto the disk, which typically has either been fabricated from discrete components or from guided wave technology, and is comprised of a mixture of micromachined and discrete elements. Assembly of the discrete elements with a high degree of alignment accuracy generally requires a substantial amount of labor and time. The lack of batch fabrication processes and the cost of such systems has been a bottleneck in the low-cost, high-yield fabrication of mass optical storage systems. In addition, the weight of the pickup head also plays a key role in the data access rate of the optical storage system. It has been shown that the data access rate is inversely proportional to the square root of the mass of the pickup head. Pickup heads consisting of individually packaged discrete elements tend to be much heavier due to the extra mass of the packages and assembly.
The previous attempts of integrating optical pickup heads using guided wave technology suffers from high coupling losses and difficulty in making high performance lenses. Another approach uses diffractive planar micro-optics, which somewhat relieves this constraint, however, the coupling of the source diode laser is difficult. This approach is also susceptible to higher optical crosstalk as light is confined to the slab and high order diffracted beams become a source of crosstalk. See T. Shiono et al., "Planer-Optic-Disk Pickup with Diffractive Micro-optics," Applied Optics, Vol. 33, at 73-50, (1986).
Therefore, what is needed is some type of optical pickup system which may be utilized with optical data storage which is a true free-space optical system that has the benefits of conventional pickup heads with discrete free-space optical elements, but which is not limited by the cost, size and difficulty of fabrication and performance limitations characteristic of the prior art technologies, but which should be manufacturable at low cost in large numbers with high yields, and would improve the optical and system performance of the optical storage system.